Method of and system for installing foundation elements in an underwater ground formation

ABSTRACT

A method of and system for installing a foundation element, in particular a (mono)pile, in a hard underwater ground formation uses a driver. The method comprises the steps of placing a foundation element on the underwater ground formation, placing a screen for reducing noise input from the driver, driving the foundation element into the ground formation by means of the driver while the screen is positioned about the foundation element, and attenuating noise transfer from the underwater ground formation to the outer wall of the screen during at least part of the driving.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national stage filing of Internationalpatent application Serial No. PCTNL2013/050263, filed Apr. 10, 2013, andpublished as WO 2013154428 A2 in English.

BACKGROUND

The discussion below is merely provided for general backgroundinformation and is not intended to be used as an aid in determining thescope of the claimed subject matter.

Aspects of the invention relate to a method of installing a foundationelement, in particular a (mono)pile, in a hard underwater groundformation by means of a driver, such as an hydraulic driver. The methodcomprises the steps of placing a foundation element on the underwaterground formation, placing a screen for reducing noise input from thedriver into the surrounding water, and driving the foundation elementinto the ground formation by means of the driver while the screen ispositioned about the foundation element. Aspects of the inventionfurther relates to a system for installing foundation elements.

SUMMARY

This Summary and Abstract herein are provided to introduce a selectionof concepts in a simplified form that are further described below in theDetailed Description. This Summary and the Abstract are not intended toidentify key features or essential features of the claimed subjectmatter, nor are they intended to be used as an aid in determining thescope of the claimed subject matter. The claimed subject matter is notlimited to implementations that solve any or all disadvantages noted inthe background.

An aspect of the invention to further improve noise mitigation, inparticular when driving foundations elements in hard subsea groundformations andor when the noise mitigation screen is made of a materialhaving a high modulus of elasticity, e.g. in excess of 100 GPa.

To this end, the method comprises attenuating noise transfer from the(underwater) ground formation to the outer wall of the screen during atleast part of the driving, preferably by at least substantiallydynamically uncoupling at least the outer wall of the screen from theground formation.

It was found that the medium, typically (sea)water, between thefoundation element, e.g. a monopile, and the screen is not the onlyacoustic transfer path and that noise generated by the pile driver istransmitted from the pile to the outer wall of the screen also via theground formation the screen is resting on or in. This is particularlytrue when the ground formation comprises hard materials, such as rock,concrete blocks (scour protection) or compacted sand, andor when thescreen is made of a material having a high modulus of elasticity. Asnoise generated by the pulse-like blows of a pile driver contains a widespectrum of frequencies, the outer wall of the screen is typicallyexcited at one or more eigenfrequencies, which dampen out relativelyslowly. By attenuating noise transfer via the ground formation, suchexcitation is avoided or at least reduced and overall acoustic radiationis further mitigated.

Attenuation is achieved passively e.g. by reducing, relative to priorart methods and systems, the stiffness (modulus) of the interfacebetween the ground formation and the lower end of at least the outerwall of the screen, or actively e.g. by establishing the maineigenfrequencies of the screen and providing means for extinguishingthese frequencies in the noise generated by the pile driver.

In an embodiment, the lower end of the outer wall of the screen ismaintained at a distance from, typically above, the ground formation,e.g. by suspending the outer wall from an inner wall of the screen (ifthe screen is double walled), from the pile (if the screen is singlewalled) or from a surface vessel. The distance is preferably in a rangefrom 5 to 200 centimeters, preferably 10 to 100 centimeters, preferably15 to 70 centimeters.

In another embodiment, the screen is at least 50%, preferably at least70% buoyant, or even has a buoyancy of 100% or more (at 100%, thedensity of screen equals the density of the (sea)water), in which casethe screen floats. In this latter configuration, it is preferred thatthe screen is held in position by one or more weights on the groundformation, such as concrete blocks or a bubble ring resting on theground formation, and connectors, such as straps or chains, connectingthe screen to the blocks or ring.

In yet another embodiment, a noise attenuating material, typically inthe shape of a ring having a circumference that corresponds to that ofthe screen, is being positioned between the lower end of the outer wallof the screen and the ground formation.

An aspect of the invention further relates to a system for installingfoundation elements, in particular (mono)piles, in a hard underwaterground formation, comprising a driver, a surface vessel, and a screen tobe placed about the foundation element to reduce noise input from thedriver. The system further comprises means for attenuating noisetransfer from the ground formation to the outer wall of the screenduring at least part of the driving.

In an embodiment, the surface vessel comprises, in addition to a cranefor securing the driver, a crane for suspending the screen andmaintaining the screen at a distance from the ground formation during atleast part of the driving. Note is this respect that known vesselscomprise a single crane for handling the pile and screen and forsecuring the pile driver during operation.

In another embodiment, the screen is at least 50%, preferably at least70% buoyant or even has a buoyancy of 100% or more and to that end e.g.comprises one or more chambers andor is made from a material having adensity lower than that of water.

In yet another embodiment, the screen is provided with a support,attached to or separate from the screen, of a noise attenuatingmaterial, preferably of a material having a modulus that is lower thanthe modulus of the material of the outer wall of the screen and lowerthan the modulus of the underwater ground formation. During driving, thesupport is positioned between the lower rim of the screen and the groundformation.

Aspects of the invention will now be explained in more detail withreference to the Figures, which show a preferred embodiment of thepresent method and system.

Within the framework of the present disclosure, the words “hard groundformation” refer to formations, natural or artificial, which, at leastat the interface with the screen comprise material having a modulus ofelasticity in excess of 10 MPa. Typical examples include, but are notlimited to, rock (modulus usually in a range from 10 to 90 GPa), denselypacked sand (modulus usually 20 to 150 MPa), and so-called scourprotection (modulus usually 10 to 90 GPa), i.e. rocks, concrete blocksor the like dumped at the driving site prior to driving and intended toprotect installed foundations element from erosion e.g. by strongcurrents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a first embodiment, wherein the noisemitigation screen is suspended from a surface vessel.

FIGS. 2 and 3 are a perspective view and a cross-section of a secondembodiment comprising an inflatable cushion.

FIG. 4 is a perspective view of a third embodiment comprising a floatingscreen.

It is noted that the Figures are schematic in nature and that details,which are not necessary for understanding the present invention, mayhave been omitted.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an embodiment of a system 1 for installing a monopile 2 inan underwater ground formation 3, e.g. a seabed. In this example, themonopile 2 has a circular cross-section and a diameter of five metersand is intended to serve, after installation, as the foundation of awind turbine.

The system 1 comprises an hydraulic driver 4 (depicted in FIG. 2), e.g.an IHC Hydrohammer S-1800, connected to a power pack on board of asurface vessel, such as a ship 5 or jack-up barge, a driver screen 6 forsecurely mounting the driver on the monopile and an anvil (hidden fromview by the driver screen) for transmitting impact energy from thedriver 4 to the monopole 2.

The system further comprises a noise mitigation screen 7, made of e.g.steel, to be placed about the foundation element to reduce noise inputfrom the driver 4 into the surrounding water. In this example, thescreen 7 comprises an inner wall 8 and an outer wall 9, i.e. is doublewalled, has a circular cross-section and an inner diameter of sixmeters. The double wall provides one or more chambers 10 for air or aporous material and renders the screen 7 50% buoyant.

The upper rim of the screen 7 is provided with a detachable extender,which is used to adjust the effective length of screen 7 to the depth ofthe water at the location where the foundation element is to beinstalled. In general, it is preferred that, once in place, thesound-insulating screen 7 extends to above the water level.

The ship 5 comprises a first crane 11 to lift and manipulate themonopile 2 and the screen 7 and a second crane (not shown) to secure thehydraulic driver 4 during driving.

Installation of a monopile is carried out for instance as follows. Thecables of the crane are attached to the upper end of a monopile storedon the deck of the ship and the monopile is lifted overboard,manipulated to an upright position, lowered onto the seabed and, ifrequired by the circumstances, allowed to penetrate the scour protectionand possibly the seabed under its own weight. At this stage, themonopile 2 is driven, e.g. by means of a vibratory device, into theseabed to a depth of some meters to further stabilize the monopole 2.

The driver 4 is positioned on top of the monopile 2 and the screen 7 islifted over the monopile 2 and the driver 4. Alternatively, the screen 7is placed and the driver 4 is subsequently placed inside the screen 7and on top of the pile 2. In this position, the pile 2 is driven to therequired depth. Finally, the driver 4 is removed, the screen 7 liftedover the pile 2 and placed back on deck or into the sea, andinstallation is completed.

A distance in a range from 20 to 100 centimeters is maintained betweenthe lower rim of the screen 7 and the seabed, thus dynamicallyuncoupling the two. As a result, substantially no noise or vibrationsare transmitted to the screen 7 via the seabed. Although noise leaksthrough the annular opening resulting from the distance, the energy ofthis noise is significantly less that that of the noise generated byexcitation of the outer wall of the screen 7 by the seabed, i.e. overallacoustic radiation is further mitigated.

In the embodiment shown in FIGS. 2 and 3, the screen 7 rests on anannular inflated cushion 12, which has a modulus of elasticity that isat least one order of magnitude smaller than the modulus of the seabedand the modulus of the screen 7. Thus, the cushion effectivelyattenuates at least the higher frequencies, i.e. serves as a low passfilter.

FIG. 4 shows a system comprising a screen 7 having a buoyancy in excessof 100%, e.g. achieved by spacing the inner and outer walls of screen 7farther apart, and a weight 13 for holding the floating screen 7 uprightand spaced from the seabed over a distance in a range from 20 to 100centimeters. In this example, the weight 13 is provided by a ring forgenerating a bubble screen inside the double walled screen 7, i.e. theouter diameter of the ring 13 is smaller than the inner diameter of thebuoyant screen 7. The screen 7 is attached to the ring 13 by means ofstraps 14.

The invention is not restricted to the embodiment described above andcan be varied in numerous ways within the scope of the claims. Forinstance, the outer wall of the screen can be suspended from the innerwall of the screen (resting on the ground formation), e.g. byinterconnections between the inner and outer walls. It is preferred thatsuch interconnections are made of or provided with a dampening material.

1. A method of installing a foundation element in a hard underwaterground formation by means of a driver, comprising: placing a foundationelement on the underwater ground formation, placing a screen forreducing noise input from the driver, driving the foundation elementinto the ground formation by means of the driver while the screen ispositioned about the foundation element, and attenuating noise transferfrom the ground formation to an outer wall of the screen during at leastpart of the driving.
 2. The method according to claim 1, comprising atleast substantially dynamically uncoupling at least the outer wall ofthe screen from the ground formation.
 3. The method according to claim2, wherein at least a lower end of the outer wall of the screen ismaintained at a distance from the ground formation.
 4. The methodaccording to claim 3, wherein the screen is suspended.
 5. The methodaccording to claim 3, wherein the screen is at least 50% buoyant.
 6. Themethod according to claim 5, wherein the screen floats and is held inposition by one or more weights on the ground formation.
 7. The methodaccording to claim 1, wherein a noise attenuating material is beingpositioned between the ground formation and the lower end of the outerwall of the screen.
 8. The method according to claim 7, wherein amodulus of the material is lower than a modulus of the material of theouter wall of the screen and lower than a modulus of the underwaterground formation.
 9. A system for installing foundation elements in ahard underwater ground formation, comprising a driver, a surface vesselconfigured to secure the driver, and a screen carried by the surfacevessel to be placed about the foundation element to reduce noise inputfrom the driver, and an assembly configured to attennuate noise transferfrom the ground formation to an outer wall of the screen during at leastpart of the driving.
 10. The system according to claim 9, wherein thesurface vessel comprises, in addition to a crane configured to securethe driver during driving, a crane configured to maintain the screen ata distance from the ground formation during at least part of thedriving.
 11. The system according to claim 9, wherein the screen is atleast 50% buoyant.
 12. The system according to claim 11, wherein thescreen comprises at least an inner wall and an outer wall.
 13. Thesystem according to claim 12, wherein the screen has a buoyancy inexcess of 100% and the system further comprises a weight configured tomaintain the floating screen upright.
 14. The system according to claim9, wherein the screen is provided with a support of a noise attenuatingmaterial to be positioned between the lower rim of the screen and theground formation.
 15. The system according to claim 14, wherein amodulus of the support is lower than a modulus of the material of theouter wall of the screen and lower than a modulus of the underwaterground formation.
 16. A noise mitigation system comprising a screenconfigured to be placed about a foundation element to reduce noise whendriven into a ground formation, and an assembly configured to attenuatenoise transfer from the ground formation to an outer wall of the screen.17. The system according to claim 16, wherein the screen is at least 50%buoyant.
 18. The system according to claim 16, wherein the screencomprises at least an inner wall and an outer wall.
 19. The systemaccording to claim 16, wherein the screen has a buoyancy in excess of100% and the system further comprises a weight configured to maintainthe floating screen upright.
 20. The system according to claim 16,wherein the screen is provided with a support of a noise attenuatingmaterial to be positioned between a lower rim of the screen and theground formation.
 21. The system according to claim 16, wherein amodulus of the support is lower than a modulus of the material of theouter wall of the screen.